Within the scope of this invention, high-precision ground shaft parts with a high surface quality are understood to be shaft-like parts whose surface quality and roundness tolerance amount to approximately 1 μm or less. Very high demands are made of the shape tolerance and surface quality for piston pins, shock absorber parts and piston rods for pressure cylinders in particular, these demands arising from the requirement for extremely reliable operation during use. Thus, for example, in the case of shaft-like shock absorber parts, sealing elements that slide on the surfaces of these shock absorber parts and must ensure a reliable seal from the inside to the outside as well as from the outside to the inside are provided on the shock absorbers. For piston pins, these high quality demands are derived from, among other things, the fact that the operating properties are exacerbated when the surface quality and the shape tolerances are lower than those given above.
It is known that the required surface quality and shape tolerance cannot be produced by grinding using known grinding wheels, which should also yield a reasonable cutting performance in addition to the high surface quality. Despite the fact that many such shaft parts are needed in large quantities, are to be manufactured inexpensively and with the shortest possible cycle time, a compromise is made between a high material removal performance and a high surface quality using known methods and machinery. This is because the required high material removal performance is achieved in a very good quality using traditional grinding machines, but the required high surface quality is achieved on an additional machine in a superfinishing process downstream from the grinding machine. Even if a combined machine, which performs the grinding in a first station and the superfinishing process in a second station, were conceivable, there would still be the major disadvantage that the workpieces must pass through at least two machining stations and thus there is a loss of manufacturing precision due to the re-chucking alone. In manufacturing on two different manufacturing machines, a) a greater space requirement is necessary, b) the costs are additionally much higher, and c) the corresponding handling systems are additionally needed between the two machines.
As a rule, a buffer storage must also be provided between the individual operating sequences, thus further increasing the cost of manufacturing.
To nevertheless be able to achieve the manufacturing costs and a short and simple flow of materials in production of the workpieces, it is known to be necessary to minimize the number of manufacturing steps. A relatively inexpensive production of the shaft-like workpieces with satisfactory accuracy, at least for a number of applications, can be achieved on surface quality and shape tolerance with the known machines and production processes. In particular, this has been possible through centerless grinding. In centerless grinding, the shaft-like components are often machined in a continuous process. However, it is impossible to achieve accuracy ranges even lower than 1 μm, as given above.
The fundamental design of such a centerless grinding machine is illustrated in a side view in FIG. 4 as an example. The workpiece to be ground rests on a support, which is also referred to as a support blade and is ground between a grinding wheel and a regulating wheel, each being in engagement with this tool. Since the grinding wheel and the regulating wheel rotate in opposite directions, the grinding wheel, which usually has a larger diameter than the regulating wheel, can grind the workpiece accordingly. Such stable centerless grinding machines are known such as the machine of the JUPITER series from the present applicant, for example.
US 2002/115391A1 describes a centerless grinding method in which continuous grinding and plunge cut grinding are combined with one another in one clamping, i.e., they are performed sequentially. The grinding disk used for this has a cylindrical zone and a conical zone and the same grinding cover.
To achieve different grinding goals—on the one hand, the greatest possible removal rate to reduce the cycle time and, on the other hand, a good surface quality—there is a known grinding tool comprised of grinding disks, which are aligned in rows side by side and are braced axially with respect to one another, as described in G 89 04 986.1, for example. This grinding tool is combined into a grinding disk package in which the grinding cover is designed differently from one disk to the next by using different grain sizes of one and the same grinding medium.
DE 295 16 264 U1 and DE 195 33 836 B4 describe a grinding wheel having—within the grinding layer—different physical properties in the axial direction and thus are adapted to the different grinding. This is achieved by the fact that the concentration of grains in the axial direction is variable, preferably being variable linearly. Thus, the wear behavior of the grinding layer is to be adapted to the allowance of the workpiece that is to be ground off.
DE 38 11 584 A1 describes a grinding wheel for deep grinding, wherein different tasks are assigned to different surface sections of the grinding wheel. Thus, with this known grinding wheel, parts of the grinding surface are designed differently, taking into account the different loads, namely with regard to the used diamond grain sizes in these sections as well as their concentration. Thus the main material removal should be performed by the part of the grinding surface of the grinding wheel that first engages in the forward direction. This should be implemented with one zone of a coarse diamond grain size and a downstream zone with fine diamond grain size.
DE 24 62 847 C2 describes a method for honing and a honing machine for carrying out the method, in which boreholes in particular are to be created by means of a tool having a conical grinding zone and a cylindrical grinding zone. This tool cuts with a high efficiency in the conical zone and creates the desired surface in the cylindrical zone, which is set for the finished dimension. The grinding layer is of a coarser quality in the area of the conical zone to achieve a greater removal performance than in the cylindrical zone.
It is thus known from the prior art described above that the concentration and size of the abrasive grains are to be varied within a single grinding wheel in accordance with the grinding requirements in order to perform different grinding tasks with a grinding wheel. In addition, it is known from this prior art that grinding wheel having a conical section may be used to achieve high removal rates and a cylindrical section to achieve the corresponding surface quality.